In an optical transmission system, it is important for an optical fiber, which is a transmission medium, to have an increased effective area so as to increase transmission quantity and a transmission distance. This is because the optical fiber having such an increased effective area can transmit intense optical signals without causing a decrease in transmission quality due to a nonlinear effect.
An example of the optical fiber having such an increased effective area is a single mode fiber, which is described in Non-Patent Literature 1. The single mode fiber described in Non-Patent Literature 1 employs a W-index refractive index profile (in which an inner part of a clad region has a refractive index lower than that of an outer part of the clad region). This allows the single mode fiber to have an increased effective area. Non-Patent Literature 1 describes that the single mode fiber employing the W-index refractive index profile can increase its effective area up to 150 μm2. An optical fiber employed in a long-distance transmission system such as a core network or a submarine system mainly employs the single mode fiber whose effective area is increased by employing the W-index refractive index profile.
However, there is limitation on increase in effective area of a single mode fiber, such as the single mode fiber described in Non-Patent Literature 1, in which the inner part of the clad region has a refractive index different from that of the outer part of the clad region by a dopant being added to the clad region. It has been therefore considered to employ a photonic crystal fiber having a further increased effective area, instead of the single mode fiber. What is meant by “photonic crystal fiber” is an optical fiber in which a clad region has a refractive index different from that of a core region by holes being formed in the clad region.
FIG. 11 illustrates a configuration of a conventional photonic crystal fiber 20 (see Non-Patent Literature 4). (a) of FIG. 11 is a cross-sectional view illustrating a cross-sectional structure of the photonic crystal fiber 20. (b) of FIG. 11 is a graph showing an effective refractive index profile of the photonic crystal fiber 20.
The photonic crystal fiber 20 has a clad region 22 in which holes 22a are periodically arranged (see (a) of FIG. 11). This allows the clad region 22 to have an effective refractive index lower than a refractive index of a core region 21 (see (b) of FIG. 11). The core region 21 and the clad region 22 are made from an identical material (for example, pure silica glass), and a refractive index difference between the core region 21 and the clad region 22 is derived from the holes 22a. Note that a difference between the refractive index of the core region 21 and the effective refractive index of the clad region 22 is hereinafter referred to as “a refractive index difference between the core region 21 and the clad region 22” in the specification.
Light is confined in the photonic crystal fiber 20 due to total reflection which is caused by the refractive index difference between the core region 21 and the clad region 22. Therefore, the photonic crystal fiber 20 can be called a “photonic crystal fiber of refractive index waveguide type”. By calling the photonic crystal fiber 20 as above, the photonic crystal fiber 20 is distinguished from a “photonic crystal fiber of photonic band gap type” that confines light by use of a photonic band gap.
Non-Patent Literature 2 describes a photonic crystal fiber which (i) can perform a single mode transmission as with a single mode fiber and (ii) has a bending loss property identical to that of the single mode fiber. The photonic crystal fiber can increase its effective area up to 157 μm2. Non-Patent Literature 3 describes a single mode fiber whose effective area can be increased up to 160 μm2 by optimizing its optical property.